This invention relates in general to threaded pipe connectors, and particularly to a riser pipe for an offshore well.
In some offshore production facilities, riser pipes are used to convey well fluids from a wellhead at the sea floor to a production platform. The riser must provide sealing against high pressure for many years. Also, the riser must be able to accommodate bending moments and fatigue due to wind, waves, and current.
Threaded production riser comprises sections of pipe, each typically having a connector welded onto each end. One connector, referred to as the pin, inserts into a receptacle referred to as a box. The pin and box have conical threaded sections that engage each other. The pin has an external shoulder that makes up against the rim of the box. In some riser pipe, the pin and box have radial interference sections for forming metal-to-metal sealing surfaces near the external shoulder of the pin and near the nose of the pin. A stress relief groove may be located at the pin base.
While there are successful prior art connectors, improvements are desirable in reducing stress amplification factors throughout the connector, improving fatigue life, and reducing the potential for damage to the thread forms.
The pipe or connector of this invention has a pin with conical first and second pin threaded sections. The threaded sections are separated by a pin intermediate section that is free of threads. The first and second pin threaded sections are formed on a single continuous conical taper.
The box has a receptacle with first and second threaded sections that are configured to make up with the first and second pin threaded sections. Similarly, the first and second box threaded sections are separated axially by an intermediate section that is free of threads. The intermediate sections engage each other with radial interference, forming a metal-to-metal seal. Metal-to-metal seals are also located at the pin nose and at the pin base.
The first and second threaded sections are formed not only on the same conical angle, but also on the same helical pattern. Although the helix is interrupted by the intermediate section, the thread form of the second section is a continuation of the helix of the thread form of the first section. Preferably, the tapered pitch line of the pin threads differs slightly from the box tapered pitch line so as to form more radial interference between the threads near the nose of the pin than at the base of the pin.
A stress relief groove is located adjacent the external shoulder of the pin. One of the metal-to-metal sealing surfaces on the pin locates between the stress relief groove and one of the threaded sections. The other metal-to metal sealing surface is at the nose of the pin. The threads are preferably V-shaped, with blunt crests. The crests of the pin threads do not contact the roots of the box threads and vice-versa. Radial interference exists between the load flanks of the threads. A shallow recess is formed on the exterior of the box to reduce stiffness.